This invention relates to a fuel supply control method for internal combustion engines, and more particularly to a method of this kind, which is adapted to control the fuel supply to the engine in accordance with a change in the operating condition of the engine when the operation of the engine shifts from an idling region to a certain low speed speed region, to thereby improve the driveability of the engine on such occasion.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine has been proposed e.g. by U.S. Pat. No. 3,483,851, which is adapted to determine the valve opening period of a fuel injection device for control of the fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
According to this proposed fuel supply control system, while the engine is operating in a normal operating condition, the air/fuel ratio of the mixture is controlled in closed loop mode wherein the value of a particular one of the above coefficients is varied in response to the output of a means arranged in the exhaust system of the engine for detecting the concentration of an ingredient in the exhaust gases so as to vary the valve opening period of the fuel injection device, whereas while the engine is operating in a particular operating region such as an idling region, a mixture-leaning region, a wide-open-throttle region and a decelerating region, the air/fuel ratio is controlled in open loop mode wherein the value of one of the coefficients corresponding to the particular operating region in which the engine is operating is set to a predetermined value so as to achieve a required air/fuel ratio best suited for the operation of the engine in the same particular operating region, threby improving the fuel consumption and driveability of the engine.
However, conventional fuel supply feedback control methods including the above proposed method are generally so arranged that when the operation of the engine leaves the idling region, the air/fuel ratio control is immediately switched over to closed loop mode from open loop mode so that the air/fuel ratio of the mixture being supplied to the engine is immediately controlled to the theoretical mixture ratio. However, when the vehicle is started to run while the engine is idling, usually the operation of the engine passes a certain low speed region adjacent the idling region, that is, a region wherein the rotational speed of the engine is lower than a value slightly higher than the idling speed and the intake pipe absolute pressure is higher than that in the idling region. If the air/fuel ratio is controlled in a feedback manner to the thereotical mixture ratio as in the conventional fuel supply control methods while the operation of the engine is passing this low speed region, there will occur a shortage in the output torque of the engine which is then in a heavily loaded state, thus resulting in a deterioration of the driveability of the engine.